This invention relates to a transducer for converting vibrations from musical instruments, especially ones having a resonant cavity, into high fidelity electrical signals for amplification and reproduction of musical sounds, and more particularly to such a transducer with an integral preamplifier contained within the enclosure of the transducer.
Piezoelectric materials have been used to produce electrical signals from vibrations in general, and of musical instruments in particular. The most common have been a type employing polarized ferrelectric ceramics, such as barium titanate or lead titanate zirconate, which is placed on the conductive wall in an enclosure with the axis of polarization in a direction normal to the wall. A conductive inertia body is then placed on the piezoelectric material and an electrical signals lead is connected to the inertia body. This lead is normally connected to the inerconductor of a coaxial cable which is plugged into a power amplifier.
The signals from a piezoelectric transducer are of sufficient amplitude to transmit over a coaxial cable of significant length to a power amplifier, but since a piezoelectric transducer has a very high impedance, typically on the order of 600 megohms, an impedance matching stage is required at the input of the power amplifier. That is not a problem since there are techniques that can be readily used to provide an impedance matching preamplifier. For example, a field-effect transistor (FET) has an input impedance of that order of magnitude so that it can be used in a source-follower configuration to provide impedance matching. The problem is that the cable from the transducer to the preamplifier introduces noise into the system. This cable noise is caused by the flexing of the cable, which changes the capacitance of the cable. The preamplifier amplifies this cable noise along with the low level transducer signal. It would be desirable to improve the signal to noise ratio of the preamplifier output by amplifying the signal and not the cable noise.
An object of this invention is to increase the signal to noise ratio of the preamplifier output for a transducer coupled to a power amplifier by a cable. These and other objects of the invention are achieved by integrating an impedance matching preamplifier for a piezoelectric transducer within the case of the transducer with a total size and weight for the integrated transducer and preamplifier so minimal as to not affect the vibration modes of the musical instrument to which the transducer is attached.
In accordance with the present invention an integrated piezoelectric sound transducer and preamplifier is provided within a sealed enclosure adapted to be mounted directly on a vibrating wall of a musical instrument, particularly the wall of an instrument having a resonant cavity. The amplifier is a single field-effect transistor (FET) connected in a common-drain (source-follower) configuration with the piezoelectric transducer connected between the gate and circuit ground inside the enclosure. The internal conductor of a coaxial cable is AC coupled to the source of the FET by a capacitor within the enclosure. The internal conductor of a coaxial cable is AC coupled to the source of the FET by a capacitor within the enclosure. DC bias voltage for the FET drain is provided through the internal conductor of the coaxial cable with a filter connected between the drain and circuit ground. The outer conductor of the cable is connected to the circuit ground of the enclosure. At the other end of the cable, the inner conductor is AC coupled by a capacitor to the input of a power amplifier, and connected to a DC power source. In that manner power for the integrated transducer and FET preamplifier within the enclosure is provided through the signal cable. Bipolar power supply may also be provided by using multiplexing switches of alternately transmit power of polarity and the other polarity. A separate diode and filter capacitor is provided to filter the power supply of the other polarity for bias voltage to be connected to the source-follower resistor.